Linear displacement transducer

There are currently no standards tests for measuring the thermal expansion of plastics in ISO or in BS 2782. In ASTM, test method D696 [124] uses a relatively thick test piece, which is placed in a chamber whose temperature can be controlled. This is shown schematically in Fig. 23. The expansion of the sample is transmitted to a remote dial gauge via a quartz rod that has a very low expansion coefficient. This same technique is applied in modern thermomechanical analyzers (TMA). but the dial gauge is replaced by linear displacement transducers or other electronic devices capable of detecting smaller dimensional changes. In turn this allows thinner specimens to be tested and permits wider temperature ranges to be examined. There are developments wnthin the ISO to provide a standard for these types of instruments. 

For creep testing, gravitational loading is usually used to provide the constant force, with specimen extension monitored by linear displacement transducers. An example is shown in Figure 6.2. 

Magnetostrictive linear displacement transducers measure the time it takes for a sonic pulse to travel Ifom a fixed source to a variable position. These newer devices are very accurate and are contact-less, offering the potential of long fife [9]. 

Mehta and Augsburger [68] later reported the mounting of a linear variable displacement transducer (LVDT) on the previously instrumented Zanasi LZ-64 machine [54] to allow the measurement of piston... 

The deformation of the sample and the electrical potential difference over the sample were measured. The electrical potential difference between both electrodes were amplified by a Unicam 9460 amplifier (Unicam, USA). During the experiments, the displacement of the piston was recorded via a linear variable displacement transducer (LVDT, Schaevitz, USA). The data acquisition sampling was 0.5 Hz. A vibrator was attached to the set-up in order to overcome the sticking of the piston to the wall. This device vibrated intermittently at 50 Hz during 1 second. The vibration started 0.5 second after the data-acquisition. Lateral forces on the piston were minimised by allowing free lateral motion of the measuring chamber floating on a silicon oil film. Further-... 

The operation of proximity sensors can be based on a wide range of principles, including capacitance, induction, Hall and magnetic effects variable reluctance, linear variable differential transformer (LVDT), variable resistor mechanical and electromechanical limit switches optical, photoelectric, or fiber-optic sensors laser-based distance, dimension, or thickness sensors air gap sensors ultrasonic and displacement transducers. Their detection ranges vary from micrometers to meters, and their applications include the measurement of position, displacement, proximity, or operational limits in controlling moving components of valves and dampers. Either linear or angular position can be measured ... 

CAL-41S Calibrator, Linear and Angular Displacement Transducers, Catalog 101, Lucas Schaevitz, Pennsauken, NJ, p. 30 (1990). 

The displacement transducers (LVDTs) which measure the absolute punch positions must also be carefully calibrated. This is particularly important when conducting studies which require accurate in-die tablet thickness measurements. Like the punch load cells, the calibration of the LVDTs are also performed in situ using a reference displacement transducer. Statistical analysis has shown that the displacement versus voltage relationship may not be linear over the full range of some transducers, and could be better described by a third-order polynomial. However, over smaller ranges, the relationship is adequately linear (24). These effects should be taken into account for a particular experiment or application as they could propagate error into in-die tablet thickness measure and subsequent. solid fraction calculations. Typically, calibration for both load cells and transducers are conducted once or twice annually. 

FIG. 5 Diagram example of a universal testing machine for bulk density testing during compression (from Thomson, 1997). LYDT, linear variable displacement transducer. 

FIG. 12 Shear tester. A constant normal load N is applied over the lid, acting like a piston. A motor moves the lower compartment, and a shear load cell measures the shear force necessary to maintain the upper compartment still. Two linear variable displacement transducers (LVDTs) measure the horizontal and vertical displacements in the cell. 

The specimens, which were placed in the hole of the bar, were compressed by the piston against the platen. Displacement in the axial direction was measured by a suitably mounted linear voltage displacement transducer (LVDT) whose core was attached to the piston (Figure 2). The calibrated output potentials of the LVDT and load cell were monitored by a fast responding dual channel recorder. 

An old but still properly working Perkin Elmer IMS-1 was adapted to perform TSD/TMA experiments. The TMS-1 is schematically drawn in Figure 6.1 together with some of the important dimensions. The sample, between the probe and the quartz glass sample holder is placed in the furnace. The furnace temperature is programmed to increase linearly with the time. The thermal expansion of the sample is measured via the probe by the linear variable displacement transducer (LVDT). A thermocouple, placed as close as possible to the sample is giving the sample tenperature information. 

A schematic of a commercial TMA instrument is shown in Fig. 16.33. The instrument consists of a dimensionally stable (with ternperamre) sample holder and measuring probe, a programmable furnace, a linear variable displacement transducer (LVDT) to measure the change in length, a means of applying force (load) to the sample via the probe (core rod, push rod), and a temperature sensor (usually a thermocouple). 

LVDT linear velocity displacement transducer MBP modified bulk polymerization... 

Figure 5 Schematic illustraticm of the MASIF (measurement and anal5rsis of surface interaction forces) SFA. The upper surface is mounted on a piezo ceramic actuator that is used for changing the surface separation the hysteresis of the piezo expansion/ contraction cycle can be accounted for hy using a linear variable displacement transducer (LVDT). The lower surface is mounted on a bimorph force sensor. (From Ref. 26, with permission.)...

Each position command issued to the motor controller caused the pneumatic pressure sensor to expand and contract to simulate respiratory movement. Note that while the motor position was controlled in a close-loop manner, the loop was not closed on the linear displacement. In a reasonable assumption, all mechanical dynamics from the motor shaft to the pressure transducer were neglected. 

The cylindrical specimen is placed into the steel load frame of the testing device, similar to the one used in the indirect tensile test. The measurement of horizontal deformation can be carried out by load transducers (linear variable displacement transducers [LVDTs]), with an arrangement similar to Figure 7.2, or by strain gauges with extensometers (see Figure 7.3). 

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